Known condensation-polymerized aliphatic polyesters, even one having a molecular weight (hereinafter, "molecular weight" means "number average molecular weight" unless stated otherwise) of about 15,000, do not have as good as properties as are possessed by polyesters having aromatic structures, such as polyethylene terephthalate (hereinafter, referred to as "PET"). It is very difficult to increase the molecular weight of an aliphatic polyester over 15,000 because further growth reaction is surpassed by decomposition reaction due to the poor heat stability of aliphatic polyesters.
Further, most aliphatic polyesters have melting points of 100.degree. C. or lower, and it would be no exaggeration to say that the range of the molecular weights thereof achieved by condensation polymerization are far from the range desirable for practical use.
Polyesters conventionally used in the polyurethane industry as raw materials for various products including moldings, formings, paints, adhesives, rigid and flexible foams, rubbers, etc., are substances more like oligomers (hereinafter, referred to as "prepolymers") having molecular weights of 1,000-3,000 and a hydroxyl number of about 50-150. Therefore, a large amount of diisocyanate, that is, 7-30 parts by weight per 100 parts by weight of polyester, must be used to achieve properties thereof suitable for practical use. However, if such a large amount of diisocyanate is used, that is, if the concentration of diisocyanate groups is significantly high, a mixture of polyester and diisocyanate of any mixing ratio always gells at a temperature of 100.degree. C. or higher, thus failing to provide a desirable product.
For example, when producing a molding by the foam or resin injection method, if polyester is mixed immediately before injection into a mold, with a liquid polyol ingredient, an isocyanate ingredient (including prepolymer type isocyanate) and a catalyst for promoting the reaction, the mixture gels and cures in a short time immediately after being injected into the mold, without forming a stable intermediate product. The resulting cure resin becomes a crosslinked type.
Isocyanate is used in products produced in the polyurethane industry as follows. For example, masked isocyanate is contained in such heat-setting type products as paints, adhesives or the like. When heated, the isocyanate is regenerated and thereby cures the products. Moisture-setting type products contain isocyanate groups at the ends of the ingredient prepolymer. It is believed that when subjected to moisture, the isocyanate groups form urethane bonds, to which another isocyanate group binds, thus curing.
Production of polyurethane rubber does not directly use isocyanate, but uses polyester prepolymers having isocyanate groups at the end thereof and, further, diols or diamines selected according to the properties, of the rubber. If isocyanate is directly reacted with polyester prepolymers, partial gelation results, thus inevitably degrading the properties required for rubber.
Fibers are produced in a manner similar to the above description although they are not simple polyesters or polyethers but so-called segmented type polymers. Similar to the above products, these polymers, both thermosetting type and thermoplastic types, are produced by using prepolymers and large amounts of isocyanates corresponding to the prepolymers.
The above products inevitably have many urethane bonds when the final structure of polyurethanes is formed. For example, if a prepolymer having a molecular weight of 2,000 is used to form polyurethanes, theoretically, a resultant polyurethane having a molecular weight of 10,000 is estimated to have 8-10 urethane bonds, and a resultant polyurethane having a molecular weight of 30,000 is estimated to have about 30 urethane bonds.
The present inventors have studied the enhancement of the molecular weight of aliphatic polyester in order to obtain a film formed mainly of a biodegradable plastic, and have found that polyesters, such as conventional polyurethane resins, which have high concentrations of urethane bonds in their molecules are not very suitable for films.
For example, if a crystalline polyester synthesized from 1,4-butanediol and succinic anhydride by a conventional method has a high concentration of urethane bonds, it becomes rigid and hard to stretch and, therefore, unsuitable for films.
An object of the present invention is to provide a method for producing a high molecular weight aliphatic polyester which eliminates the above-stated problems of the conventional art and is suitable for various uses, such as films, moldings, formings, fibers, etc.